1. Field of the Invention
The present invention relates to circuitry for detecting deviation in frequency of an input signal with respect to a reference signal. In particular, the present invention relates to digital circuitry for demodulating frequency shift key data transmissions.
2. Description of the Prior Art
With the ever-increasing use of digital computers and other digital electronic circuitry, the transmission of digital data has become extremely important. One digital data transmission format which has been utilized is frequency shift key (FSK) transmission. An FSK transmitter modulates a reference frequency signal based upon the data to be transmitted so that the transmitted signal has a frequency (f.sub.FSK) which is either slightly greater than or slightly less than the frequency (f.sub.0) of the reference signal (i.e. f.sub.FSK =f.sub.0 .+-..DELTA.f). The frequency of the transmitted FSK signal, therefore, represents either a digital "1" or a digital "0". An FSK receiver receives and demodulates the transmitted FSK signal to produce a serial data stream based upon the frequency of the FSK signal representing each bit of the data stream.
Narrow band FSK transmission, in which the frequency deviation of the FSK modulated signal from the reference or center frequency is very small, offers significant advantages. Because the FSK modulated signal is present only within a narrow frequency band, the narrow band FSK receiver can use a very high Q filter to remove all noise contained in the frequency spectrum except for the narrow band containing the FSK signal. High noise immunity, therefore, can be achieved by using narrow band FSK transmission.
There is a continuing need for improved FSK receiver circuitry which is capable of detecting narrow deviation in frequencies of the input signal with respect to a reference frequency, which requires short operating times, and which has good noise immunity. In addition, due to the ever increasing use of integrated circuits, there is need for FSK receiver circuitry which utilizes a minimum of components and which is completely integrable on a monolithic integrated circuit chip.